The present invention relates in general to burnable absorbers, also referred to as burnable poisons, for nuclear reactors, and more particularly, to a burnable absorber assembly for use in a nuclear reactor core of fuel assemblies having a plurality of guide thimbles for each receiving a reactor control rod and which burnable absorber assembly is constructed and arranged to be inserted within the guide thimbles for controlling the reactivity and ultimately to extend the life of the fuel assemblies.
The process of nuclear fission involves the disintegration of fissionable nuclear fuel material into two or more fission products of lower mass number. Among other things, the process also includes a net increase in the number of available free neutrons which are the basis for a self-sustaining reaction. When a reactor has operated over a period of time, the fuel assembly with fissionable materials must ultimately be replaced due to depletion. Inasmuch as the process of replacement is time consuming and costly, it is desirable to extend the life of a given fuel assembly as long as practically feasible. For that reason, deliberate additions to the reactor fuel of parasitic neutron-capturing elements in calculated small amounts may lead to highly beneficial effects on a thermal reactor. Such neutron-capturing elements are usually designated as burnable absorbers if they have a high probability or cross-section for absorbing neutrons while producing no new or additional neutrons or changing into new absorbers as a result of neutron absorption. During reactor operation the burnable absorbers are progressively reduced in amount so that there is a compensation made with respect to the concomitant reduction in the fissionable material.
The life of a fuel assembly may be extended by combining an initially larger amount of fissionable material, as well as a calculated amount of burnable absorber. During the early stages of operation of such a fuel assembly, excessive neutrons are absorbed by the burnable absorber which undergoes transformation to elements of low neutron cross-section which do not substantially affect the reactivity of the fuel assembly in the latter period of its life when the availability of fissionable material is lower. The burnable absorber compensates for the larger amount of fissionable material during the early life of the fuel assembly, but progressively less absorber captures neutrons during the latter life of the fuel assembly, so that a long life at relatively constant fission level is assured for the fuel assembly. Accordingly, with a fuel assembly containing both fissionable material and burnable absorber in carefully proportioned quantity, an extended fuel assembly life can be achieved with relatively constant neutron production and reactivity. Burnable absorbers which may be used include boron, gadolinium, samarium, europium, and the like, which upon the absorption of neutrons result in isotopes of sufficiently low neutron capture cross-section so as to be substantially transparent to neutrons.
The incorporation of burnable absorbers in fuel assemblies has thus been recognized in the nuclear field as an effective means of increasing fissionable material capacity and thereby extending reactor core life. Burnable absorbers are used either uniformly mixed with the fissionable material, i.e, distributed absorber, or are placed discretely as separate elements in the reactor core. Thus, the net reactivity of the reactor core is maintained relatively constant over the active life of the reactor core. However, the use of burnable absorbers either directly with the fissionable material or as separate elements in the reactor core has its limitations in extending the life of a given fuel assembly beyond its originally designed fissionable material replacement cycle. For example, nuclear reactors used for power generation have typically been designed for twelve month fissionable material replacement operating cycles. At the end of the operating cycle, the nuclear reactor core is required to be refueled by replacement of about one-third of its fuel rods containing fissionable material so as to extend the operating cycle of the reactor for an additional twelve months. The process of fissionable material replacement is not only time consuming, for example, taking as much as six weeks, but is also costly in terms of lost power generation. As a consequence, there has been a need for developing the means by which the operating cycle of these power generating nuclear reactors can be increased to, for example, eighteen months without the requirement for fissionable material replacement.
Although the use of burnable absorbers either mixed with the fissionable material or as separate elements in the reactor core has been known to extend the reactor core life and operating cycle, the use of such burnable absorbers in this manner is limited, particularly with regard to existing nuclear reactors designed for a predetermined operating cycle. For example, the use of a burnable absorber in the fuel rods require a corresponding displacement and loss of fissionable material, typically greater than about four percent. As the burnable absorber is therefore not removable from the fuel rod during operation of the nuclear reactor, its presence penalizes core thermal margin by increasing the linear heating rate, i.e., in kilowatts per foot, and heat flux by the percentage of the fissionable material displaced. As an alternative approach, the fissionable material within each of the fuel rods may be enriched to a greater amount so as to extend the operating cycle of the nuclear reactor. In order to compensate for the use of the enriched fissionable material, a plurality of burnable absorbers in the form of rods are required to be inserted throughout the reactor core. This is not always possible as a number of presently installed nuclear reactors use burnable absorbers which displace the fissionable material in the fuel rods and therefore do not provide for the installation of these burnable absorber rods. Furthermore, the enrichment of the fissionable material in these existing reactors is limited as the resulting higher heat generated per fuel rod often cannot be adequately controlled. Thus, this limitation on the ability to enrich the fissionable material within the fuel rods, as well as the required need for burnable absorption rods, which in themselves displace fuel rods, result in the operation of a nuclear reactor at less than its maximum rated power output. This reduction in power output results in a financial loss which generally cannot be recouped by the extended life of the nuclear reactor operating cycle.
Accordingly, it can be appreciated that there is an unsolved need for a burnable absorber assembly which may be utilized with existing, as well as future nuclear reactors, for example, of the power generation type, which extends their operating cycle and which eliminates the above-noted thermal penalty, in addition to reducing fissionable material cycle costs.